1. Field of the Invention
This invention relates generally to binder resins for resin transfer molding preforms and, more particularly, to powdered binder resins that are highly compatible with the subsequently injected matrix resin. This invention also relates to preforms assembled with the powdered binder resins and a method for preparing such preforms.
2. Description of the Related Art
Resin transfer molding (RTM) is a process used to fabricate fiber-reinforced composite articles. The process involves two basic procedures: fabricating a fiber preform in the shape of the finished article and impregnating the preform with a thermosetting resin, commonly called the matrix resin. The resulting fiber-reinforced composite article displays high strength and low weight. Such articles are used in the aerospace industry and for other applications requiring low weight, high strength materials.
The first procedure in the RTM process is to fabricate the fiber preform in the shape of the desired article. The preform generally comprises a plurality of fabric layers or plies that impart the desired reinforcing properties to the ultimate composite. As an example, the fabric plies may comprise graphite, aramid or glass fibers. Generally, the techniques used to form the preform are borrowed from the garment industry. For example, the fabric plies may be cut according to a predetermined pattern, loosely bonded or stabilized so that the plies maintain the desired shape, and laid up on a mandrel or placed in a mold having the desired shape.
Once the fiber preform has been fabricated, it is placed in a closed cavity mold for the second procedure in the RTM process. The mold is closed and the matrix resin (typically an epoxy) is injected under pressure to initially wet and eventually impregnate the preform. The matrix resin initially has a very low viscosity to enable it to thoroughly impregnate the preform. The temperature of the mold is then set to the cure temperature of the matrix resin, thereby causing the resin to increase in viscosity and, ultimately, to solidify. A finished fiber-reinforced composite article typically comprises between approximately 50 volume % and 65 volume % fibers.
One of the bottlenecks in the RTM process is stabilizing the preform prior to placing it in the closed cavity mold. Stabilization is necessary to ensure that the fabric plies maintain the desired shape and orientation until they are impregnated with the matrix resin. Stabilization also prevents the fabric plies from unraveling along cut edges. Common methods used to stabilize the preform include stitching, adhesively bonding the fabric plies with a nonreactive solid or viscous liquid binder resin, and hot iron tacking. These methods are both labor and time intensive.
Stitching can be a very effective means of stabilizing the preform; however, stitching is difficult to perform after the fabric plies have been laid up on a mandrel or placed in a mold. Moreover, stitching is not conducive to fabricating a preform having a complex shape because stitching tends to make the fabric plies too rigid to bend easily.
Adhesively bonding the fabric plies with a binder resin is a promising approach. However, some currently employed binder resins may adversely affect the mechanical properties of the subsequent composite article. For example, if the binder resin is not highly compatible with the matrix resin, the ultimate composite may display a reduced glass transition temperature and/or a broadened glass transition temperature as well as changes in tensile modulus and moisture absorption.
Hot iron tacking is more amenable to fabricating preforms that have a complex shape than are other methods. However, it may be more labor intensive and time consuming if the preform includes many plies. In one approach, a thermoplastic polymer is disposed between each fabric ply and is melted with a hot iron. The polymer is allowed to resolidify, thereby bonding adjacent plies together. Tacking must be done one ply at a time because heat transfer is by conduction only. (Many reinforcing fabrics are poor heat conductors.)
Consequently, it would be desirable to have a method for assembling resin transfer molding preforms that did not involve stitching, a reduction in the mechanical properties of the ultimate composite article, or hot iron tacking.
U.S. Pat. No. 4,992,228, "Method for Preparing Preforms for Molding Processes," issued Feb. 12, 1991, to H. G. Heck et al. and U.S. Pat. No. 5,071,711, "Preforms for Molding Processes, "issued Dec. 10, 1991, also to H. G. Heck et al. disclose resin transfer molding preforms and a method for preparing the same. The preforms are prepared by melting a solid, nonsintering, thermoplastic-like resinous compound on the surface of a substrate material. Once the resinous compound has been cooled, one or more plies of the coated substrate material are assembled into the desired shape, heated to a temperature above the melting temperature of the resinous compound, and then cooled to a temperature below the melting point of the resinous compound so as to provide a preshaped preform for use in resin transfer molding. The thermoplastic-like resinous compound can be melted or solidified at will by raising or lowering the temperature of the substrate and is substantially free of any compound that would serve as a crosslinker therefore.
U.S. Pat. No. 4,988,469, "Method of Fabricating Fiber Reinforced Composite Articles by Resin Transfer Molding," issued Jan. 29, 1991 to R. T. Reavely also discloses a multistep process. A plurality of dry fabric plies are stacked, formed into the desired shape, and stabilized so as to provide a dry fiber preform. The preform is placed in a resin transfer mold and injected under pressure with a thermosetting resin that impregnates the stabilized fiber preform. The preform is stabilized by disposing a polymeric binder between adjacent fabric plies and melting the binder by directing a stream of heated air through the fabric plies so as to bind them together.
U.S. Pat. No. 4,684,678, "Epoxy Resin Curing Agent, Process, and Composition," issued May 30, 1985 to W. J. Schultz et al. discloses an epoxy resin composition comprising at least one aromatic polyepoxide and at least one 9,9-bis(aminophenyl)fluorene curing agent. The compositions are reportedly useful for impregnating woven and nonwoven webs for composite articles.